Hyperspectral imaging is one of the most promising new remote sensing technologies emerging from the laboratory in recent years. This technology involves the use of imaging spectrometers to remotely measure two dimensional variations in surface spectral reflectivity of an area of interest. These spectral measurements are compared against a known set of template or reference spectra.
Traditional single or multispectral systems tend to rely on spatial techniques which often are not satisfactory when performed automatically. In view of this, highly trained and alert human interpreters are often required. As an example, consider the case of a reconnaissance system being flown over a forest in an attempt to detect camouflaged tanks. A single or multispectral system would need sufficient resolution to obtain texture and shape information. Further, the processor or human operator would need to search the scene subregion by subregion looking for a partially obscured rectangle in an arbitrary orientation with the right texture.
It is apparent from recent experimental projects involving imaging spectrometers that spectrometers will play an important role in the future of remote sensing. For instance, these spectrometers will be able to provide the necessary digital spectral imaging crucial for accurate geological, geographic, and environmental surveys, extent of pollution and blighted crops. Thus, present day spectrometers, as they become cheaper and easier to operate, will become useful for measuring forestation, population growth, and other geographic targets.
In order to accurately measure geographic targets, such as hydrological, agricultural or ecological surveys in an economically feasible manner, operation of current spectrometers must be simplified and made more robust and, as important, these systems must be made significantly less expensive. This is because current spectrometers are optimized for controlled laboratory use and do not provide the required ease of use, reliability and flexibility for spectral digital imaging as a stand alone field unit system. Many of these imaging spectrometers are actually used for various specific chemical or research applications to simultaneously acquire only several spectra. Such limited spectral sensing is not useful for widespread geographic coverage.
Imaging spectrometers fall within one of two categories (1) those that look simultaneously at the target in a number of specific wavelength intervals, and (2) those which repetitively scan a given wavelength interval. Such scanners are known as spectral range spectrometers. Spectrometers are further categorized according to their spectral sorting capabilities, i.e., conventional and interferometric spectrometers, wherein conventional spectrometers use either gratings or prisms as a dispersing means. A basic spectrometer will further consist of many mirror coatings, gratings, and detector options to provide the necessary flexibility.
There are expensive research instruments which have the spectral resolution, range, and signal-to-noise ratios (SNR) to perform useful discriminations. These systems, are expensive, difficult to operate, unreliable and not suited to field operations. Furthermore, data is generally processed off-line by highly trained personal and, is thus, not quickly available.
Other available instruments are less expensive and have limited near real-time and real-time discrimination capabilities. Their focal planes (whether intensified or unintensified) and optics do not provide adequate SNR and optical performance to do economically valuable tasks. There is a third class of instruments which are a variety of imaging spectrometers generally marketed for chemical applications. These instruments are inexpensive but of poor optical imaging quality and contain inappropriate hardware and no software for any type of terrestrial imaging.
Most airborne imaging spectrometers use a reflection or transmission grating for the dispersive element because the gratings have linear dispersions as a function of wavelength, whereas prism spectrometers have a non-linear dispersion that is sensitive to changes in operational conditions such as altitude, temperature, and humidity. Thus, prism spectrometers are typically more sensitive to climatic changes, thereby resulting in poorer data quality as compared to grating spectrometers, unless continuous, real-time calibrations are performed.
Several alternate spectrometer configurations and techniques have begun to emerge in recent years including multiplexing of the signals from a large number of entrance and exit slits to overcome the limited field-of-view and lower optical efficiency of the single slit spectrometer. Other advances in spectrometers involve the dispersion and separate coding of various portions of a spectrum in the focal plane of the spectrometer. After this is accomplished, the radiation is guided back through the dispersing system and reassembled and focused on a single detector. Spectral information is then recovered using frequency spectrum extraction techniques.
Other examples of current systems include the use of acousto-optic tunable filters (AOTF) which operate from the ultra-violet to the infra-red regions of the optical spectrum. In general, an acoustic transducer and an acoustic absorber are bonded to opposite ends of an acoustic-to-optic crystal. The transducer converts a high frequency rf signal into a pressure wave which then propagates laterally through the acoustic-to-optic crystal. The resulting frequency of the standing waves act as a tunable grating to disperse the incoming energy into spectral components. The absorber then eliminates any acoustic reflections which would corrupt the primary rf signal. The AOTF technology is typically utilized in micro applications such as imaging a cross section of human epithelia cells as disclosed in U.S. Pat. No. 5,528,368 to Lewis et al. ("Spectroscopic Imaging Device Employing Imaging Quality Spectral Filters").
A macro spectrographic application for geographic surveys and similar applications is disclosed in U.S. Pat. No. 5,371,358 to Chang et al. wherein an improvement to an optical image assembly is shown. The improvement comprises a method for absolute radiometric calibration during data acquisition for a spectrographic detector array. The array is arranged in a matrix of rows and columns comprising a first and second linear detector array proximate to a first and second column of detector arrays, respectively. A cover for deflecting light from the first linear array in order to establish a dark current reference level is provided. Illumination is then provided from a calibrated light source to the second linear array. In this manner a sequential sampling of the detected output of the linear arrays are temporarily calibrated by the associated sensor elements in the linear detector arrays.
U.S. Pat. No. 5,475,212 to Nelson et al. discloses a "wavelength domain scanned imaging" spectrometer. This spectrometer is quite different from a typical CCD spectrometer in that beams of different wavelengths are simultaneously scanned across an area detector, and line readouts are synchronized to provide images at the different wavelengths. In a CCD spectrograph, also known as a "time domain image" spectrometer, an image is scanned across a CCD and the line shift and readout of the area CCD are synchronized with the rate of image motion. This spectrometer requires a very stable line of sight as spatial-spectral mixing occurs when the field of view moves in any direction other than along the velocity vector of the flight platform.
Currently, none of the disclosed systems provide the necessary combination of high image quality along with operational robustness and flexibility needed to perform extended surveillance and monitoring studies in the field. Further, present geographic systems have a tendency to use large pixel focal planes which by nature require a high dispersion grating and associated long focal length optics that results in instruments with large physical dimensions and correspondingly large weight. This leads to a requirement for larger, higher performance aircraft platforms as well as associated higher operating costs. The above systems also use custom built optics, cooled focal planes, software oriented to off-line data processing and expensive digital recorders which translate into very high cost, reliability problems and operational complexity.